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Fair value of financial instruments and marketable securities (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities $ 130,730 $ 127,053
Recurring basis | Total
   
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 130,730 127,053
Warrant liability 113 58
Recurring basis | Significant other observable inputs (level 2)
   
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 130,730 127,053
Recurring basis | Significant unobservable inputs (level 3)
   
Financial assets and liabilities measured at fair value on recurring basis    
Warrant liability $ 113 $ 58